The present invention relates broadly to the measurement of emission levels in a gas stream, and more particularly to a system and method for monitoring the level of a VOC or other vapor or solid contaminant being emitted in a vented gas stream or the like.
The widespread use of solvents and the like in industrial applications has resulted in increased emissions of volatile organic compounds (VOCs) into the atmosphere, giving rise to environmental concerns and prompting stricter legislative controls on these emissions. One such control promulgated at the federal level is the Clean Air Act of 1977. This Act, along with its 1990 and 1991 amendments, mandates that industry monitor air pollutants, maintain emission records, and make such records available to officials of the Environmental Protection Agency, the federal agency charged with enforcement of the Act. As a consequence, manufacturers of pharmaceuticals, coated products, textiles, and polymer composites and foams, as well as hydrocarbon producers and distributors, have sought improvements both in the methods for removing VOCs from process gas streams, and for monitoring the levels of VOCs that are emitted to ensure that such levels are within prescribed limits.
One method for removing VOCs from process gas streams is disclosed in U.S. Pat. Nos. 4,480,393 and 5,152,812. By first concentrating the condensable vapors contained in a process gas stream carrier and then employing refrigeration condensation to effect their recovery, the method operates more energy efficiently than those methods previously known. Particularly, this method entails first passing a process gas stream carrying a condensable organic compound such as a solvent through, for example, a packed carbon bed which adsorbs the solvent vapor such that solvent is accumulated in the bed and a solvent-free process gas stream may be exhausted or recycled back to the process. Upon becoming saturated with adsorbed solvent, the bed is regenerated by the circulation therethrough of a heated inert gas or air stream to vaporize the solvent. Once vaporized, the solvent is carried by the gas stream from the bed to a refrigeration/condensation recovery system. Therein, the vapor-laden gas stream is passed through a turbine expander to thereby effect an expansive cooling at pressures near atmospheric to temperatures generally well below the boiling points of the solvents to be recovered. Separation of the condensable solvent vapors from the relatively non-condensable inert gas stream carrier subsequently may be effected with the recycling of recovered solvent back to the process and the recycling of inert gas back to the packed bed to continue its regeneration.
With improvements in the methods for removing VOCs from process gas streams, have come calls from industry for similar improvements in the methods heretofore known in the art for monitoring the level of VOC emissions. Prior methods have involved calculating the amount of the emission as a product of the flow rate of the gas stream being vented and the average concentration of the contaminant in the gas stream. Accordingly, both the flow rate of the gas stream and the concentration of the contaminant in the gas heretofore have had to be measured such that an average concentration could be determined. However, in most industrial processes, both the gas flow rate and contaminant concentration are time dependent. Thus, the prior methods were susceptible to concentration peaks and to unsteady-state flow in yielding sometimes spurious results. The preferred method, in contrast, would be unaffected by time dependent or localized concentration gradients, or by erratic flow. The preferred method additionally would be adaptable to the measurement of both vapor and solid phase emission, would not introduce a pressure drop or other obstruction into the flow of the emission, which pressure drop or obstruction itself would affect the emission rate or level being monitored.
In view of the foregoing, it may be seen that there has existed and remains a need for improvements in measuring the emission levels of process gas streams and the like.